From the standpoint of a political scientistwho has studied democracies around the world, Donald Trump from the beginning has seemed to pose a threat to the American system unlike any president in living memory, willing to use his mandate as a democratically elected leader to weaken the check-and-balance institutions of the American constitutional system. In January 2017, however, as Trump took power, I called for caution in a Politico Magazineessay because I expected that our institutions would hold and that divisions among Republicans would prevent the worst.

In his second year, we have now seen how those institutions are faring. In one sense, the report card is good: The fact that special counsel Robert Mueller’s probe has continued to produce indictments in the face of direct attacks by Trump and his supporters suggests that our institutions are still doing a reasonable job in constraining power. I also noted that Trump would soon discover the federal government doesn’t control nearly as much as he thought—states and cities would provide yet another bulwark against the strongman rule so many feared. This has proved true.

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But there’s another, more important check that is more worrisome: the voters. In the American system, the ultimate check on power is not a rival branch of government, but an election. Check-and-balance institutions depend on the faith of political actors and the general public, and the biggest threat to American democracy today is the breakdown of consensus over the fundamental legitimacy of those institutions. Trump has already succeeded in convincing a large part of the Republican electorate that the FBI and the Justice Department, institutions fundamental to maintaining their liberties, are deeply corrupt.

The midterm elections in November will be some of the most important in American history. Democratic publics can and do make mistakes, but if they fail to correct those mistakes, they are in deep trouble. If the American people repudiate Trump by giving the Democrats control over the House of Representatives and (as a long shot) the Senate, then bodies like the House Intelligence Committee could start doing serious work again, holding investigations, publishing documents and grilling administration officials. If, by contrast, American voters allow the Republicans to keep control over both houses of Congress, Trump will trumpet this as legitimation of all he has done to protect himself and to undermine our institutions. Should that happen, American democracy will be in real trouble indeed.